1. Field of the Invention
Embodiments of the present invention are directed to compositions and processing methods of rare-earth vanadate based materials that have high emission efficiency in a wavelength range of 480 to 700 nm with the maximum intensity at 535 nm (bright yellow) under UV, X-ray and other high-energy irradiation. The materials of the invention can be used in a form selected from the group consisting of bulk, sheet, film, ceramic, single crystal, glass, and composite forms.
2. State of the Art
Luminescent materials play an important role in applications for color television, energy-saving fluorescent lamps, LEDs and other display-systems and devices. These phosphors are characterized by light output (energy-conversion efficiency), color, thermal stability, response time, decay time. Scintillators are phosphors that show luminescence under X-ray radiation. They are commonly used in today's X-ray imaging detectors for medical diagnostics, security inspection, industrial non-destructive evaluation (NDE), dosimetry, and high-energy physics.
Recently, there has been an increasing demand for transparent, high atomic density, high speed and high light-output scintillator crystals and ceramic materials as detectors for computed X-ray tomography. Many transparent ceramics such as (Y,Gd)2O3:Eu3+, Gd2O2S:Pr,F,Ce have recently been developed for this purpose. However their slow response and lack of single crystal form have limited their applications for X-ray Explosive Detection systems and X-ray panel displays.
The currently used scintillators for X-ray Explosive Detection system are mainly CsI and CdWO4 single crystals. Even though CsI exhibits a high light output, CdWO4 crystals are more popular for X-ray Explosive Detection due to slow scan speed associated with afterglow problem for CsI. As listed in Table 1, low light output is a disadvantage for CdWO4.
TABLE 1The characters of the X-ray scintillators currently used in ESD and Panel Display Emiss.Rel.AfterX-raywave-lightglowRadiationScintillatorslengthoutput(%@Damagefor FPDDensity(nm)(%)50 ms)(%)ToxicityStabilityCsI:Tl4.55501000.3+13.5Tl: toxicMoisturesensitiveCdWO47.9530~30<3 × 10−6−2.9ToxicStableGd2O2S:Pr, Ce7.34550–650~60<0.01<−3.0CorrosiveStable
Bismuth as a tri-valent primary activator in YVO4 is known to have high emission efficiency, exhibiting broad-band luminescence, and is also known to improve emission when europium is used as a sensitizer if co-doped in ppm levels. Bismuth substituted vanadates exhibit superior advantages in that they display short luminescence decay times of a few μs in comparison to the rare earth elements (such as Eu3+, Nd3+, Tb3+ doped scintillators) which have decay times on the order of about 1 ms. Scintillators with bismuth as an activator are contemplated in this disclosure to be ideal materials of choice as detectors in X-ray tomography. Though bismuth has desirable qualities, it has the disadvantage of evaporating easily at high temperatures in the process of making such phosphors, and thus deviations of stoichiometry that leads to the fluctuation in properties results. Therefore, it is critical to develop a process to maintain the bismuth concentration at desired levels during the material synthesis. Embodiments of the present invention are directed to novel bismuth containing phosphors, as well as methods of their preparation.